The theory of the infrared Schottky-barrier detector with optical cavity is described by Hammam Elabd and Walter F. Kosonocky in "Theory and Measurements of Photoresponse for Thin Film Pd.sub.2 Si and PtSi infrared Schottky-Barrier Detectors With Optical Cavity ", RCA Review Vol. 43, December 1982, pages 569 to 589. A typical example of the infrared Schottky-barrier detector with optical cavity is illustrated in FIG. 1 of the drawings. The infrared detector illustrated in FIG. 1 is of the back illuminated structure type with a Schottky contact between a metal and a p-type semiconductor material and comprises a p-type monocrystalline silicon substrate 1. The silicon substrate 1 has polished surfaces on both sides thereof, and one of the polished surfaces provides a light incident surface coated with an antireflection material 2. On the other polished surface is formed a platinous mono-silicide (PtSi) film 3 which serves as a photoelectric converting region with a Schottky-barrier formed at a boundary surface between the p-type silicon substrate 1 and the platinous silicide film 3, and the photoelectric converting region is surrounded by an n-type impurity region 4 serving as a guard ring for reduction in edge effect. The other polished surface is covered with a thermally grown silicon dioxide (SiO.sub.2) layer 5 which defines the photoelectric converting region. On the entire surfaces of the silicon dioxide layer 5 and the thermally grown silicon dioxide layer 5 is deposited a dielectric layer 6 of silicon oxides (SiO, SiO.sub.2 and so on) or, alternatively, of silicon nitrides (SiN, Si.sub.3 N.sub.4 and so on) by using a chemical-vapor-deposition technique. For enhancement of optical absorption, a metallic mirror 7 is formed on the dielectric layer 6, then an optical resonance takes place in a multiple layer structure consisting of the p-type monocrystalline silicon substrate 1, the platinous silicide film 3, the dielectric layer 6 and the metallic mirror 7. The dielectric layer has a thickness optimized in such a manner that the platinous silicide film 3 is located at one of antinodes of a steady wave produced by an infrared ray with the major wavelength within a detectable range. An n-type heavily doped impurity region 9 is formed in the silicon substrate 1, and the heavily doped impurity region 9 is contact with the platinous silicide film 3 to form an ohmic contact therebetween. The heavily doped impurity region 9 in turn is contact with an aluminum wiring layer 10 in ohmic manner, so that electric charges produced by the photoelectric conversion are effectively extracted from the platinous mono-silicide film 3 to the outside through the n-type heavily doped region 9 and the aluminum wiring layer 10. On the entire surface of the structure is deposited a passivation film 11 which prevents the multiple layer structure from physical and chemical damages. In this prior-art example, the silicon substrate 1 is of the p-conductivity type , but the silicon substrate 1 and the impurity regions 4 and 9 may be of the n-conductivity type and of the p-conductivity type, respectively, in a metal-n-type semiconductor Schottky-barrier implementation. Moreover, a front illuminated structure is formed without the anti-reflection material 2 and the metallic mirror 7, but the dielectric layer 6 serves as not only a passivation film but also an anti-reflection film.
The fabrication processes used for silicon integrated circuits can be adopted to form these prior-art infrared Schottky-barrier detectors, and it is relatively easy to form the photoelectric converting regions in array. Then, the structure illustrated in FIG. 1 is appropriate to a photo detecting array of a solid-type infrared image sensor. In the photo detecting array, electric charges produced by the photoelectric conversion are not directly supplied to an aluminum wiring layer corresponding to the aluminum wiring layer 10, but are read out to a charge-coupled-device or, alternatively, a MOS structure scan-and-reading-out unit.
As to the operating condition, the PtSi infrared Schottky-barrier detector illustrated in FIG. 1 is used in a temperature around a liquid nitrogen, because a large amount of dark current flows in an operation at a room temperature due to the relatively low Schottky-barrier height ranging between 0.20 eV and 0.25 eV which results in a large number of thermally excited carriers flowing across the low Schottky-barrier. Of course, the amount of the dark current is relationally increased or decreased depending upon the Schottky-barrier height, then the operating temperature may be lower if the Schottky-barrier is decreased in height but higher if the Schottky-barrier is increased in height.
Subsequently, description is hereinunder made for the photoelectric conversion phenomenon with reference to FIGS. 2 and 3. FIG. 2 shows an energyband diagram showing a Schottky-barrier formed at a boundary surface between a metal layer and a p-type semiconductor material, and, on the other hand, FIG. 3 shows an energyband diagram showing a Schottky-barrier formed at a boundary surface between a metal and an n-type semiconductor material. Both of the energyband diagrams are applicable to the back illuminated structures, respectively, however incident rays are illuminated upon front sides opposite to the back sides, respectively, if the detectors are of the front illuminated type. Upon illumination of the rays, the photons having energies sufficient to excess the energyband gap Eg of the semiconductor material are liable to be absorbed in a region of the semiconductor material located in the vicinity of the incident surface. However, if the photons have lower energy levels as those of the infrared rays, there is negligible probability of absorption of energy which results in transition of electrons from the valence band to the conduction band, so that the photons pass through the semiconductor material and reach the metal region without substantial loss. In the metal region, all of the energy levels under the Fermi level Ef are occupied by electrons, however when the photons of the infrared rays reach the metal, some of the electrons in the valence band are exited with the photon energies hv ( where h is Planck's constant and v is the light frequency ), so that the exited electrons transit to the conduction band as hot electrons EL, leaving hot holes HL in the valence band. These hot electrons EL and hot holes HL travel in the metal region until recombination, and the movements thereof isotropically take place. Then, some hot holes HL reach the boundary surface between the metal region and the p-type semiconductor material in the structure having the energyband shown in FIG. 2, and the hot holes HL with component energies sufficient to exceed the Schottky-barrier E.sub.SB are injected from the metal region into the p-type semiconductor material. In other words, a hot hole can be injected from the metal through the Schottky barrier to the p-type semiconductor material if the hot hole has an energy larger than the Schottky barrier height and the component quantity of motion perpendicular to the boundary surface between the metal and the p-type semiconductor material the converted energy of which is larger than the Schottky barrier height. Thus, the injected hot holes and the leaving hot electrons participate in electric currents capable of forming electric signals. The energy of the hot hole is measured with respect to the Fermi level Ef and has positive values in the lower region under the Fermi level in FIGS. 2 and 3.
On the other hand, in the structure with the energy band shown in FIG. 3, some hot electrons similarly reach the boundary surface between the metal region and the n-type semiconductor material, and the hot electrons with component energies sufficient to exceed the Schottky-barrier E.sub.SB are injected from the metal to the n-type semiconductor material, leaving the hot holes in the metal. In other words, a hot electron can be injected from the metal through the Schottky barrier to the n-type semiconductor material if the hot electron has an energy larger than the Schottky barrier height and the component quantity of motion perpendicular to the boundary surface between the metal and the n-type semiconductor material the converted energy of which is larger than the Schottky barrier height. These injected hot electrons and the hot holes participate in electric currents representative of electric signals as in the similar manner to that shown in FIG. 2. The energy of the hot electron is also measured with respect to the Fermi level Ef, but the positive area extends in the opposite direction to that of FIG. 2.
In the case where the structure is of the front illuminated type or has a thin semiconductor layer with transparency to the incident rays, the uppermost energy level of available rays is larger than the energy bandgap Eg, so that free electrons and holes produced in the semiconductor material by the photoexciting also participate in the electric currents representative of the electric signal.
Further, if the thickness of the metal is smaller than the traveling distance of each hot hole in the (case of FIG. 2) during the lifetime or the traveling distance of each hot electron (in the case of FIG. 3) during the lifetime, some hot holes or some hot electrons move toward the dielectric material and are reflected on the boundary surface between the metal region and the dielectric material, then traveling in the opposite direction toward the boundary surface between the metal and the semiconductor. If the hot holes or the hot electrons have energy levels sufficient to exceed the Schottky-barrier, these hot holes or the hot electrons are injected into the semiconductor material and participate in the electric currents. On the other hand, the hot holes and the hot electrons with insufficient energy levels are reflected on the boundary surface between the metal and the semiconductor material and repeat the reflection between the two boundary surfaces, however an incidence of angle is gradually varied during the reflection between the two boundary surfaces and, finally, allows some of the hot holes or the hot electrons to have the energy components each sufficient to exceed the Schottky-barrier. In detail, it is necessary for a hot hole or an hot electron capable of being injected into the semiconductor material during reflections between the boundary of the metal and the semiconductor material and the boundary of the metal and the insulating material to have an energy larger than the Schottky barrier height. Although the hot hole or the hot electron has an energy larger than the Schottky barrier height, the component of quantity of motion perpendicular to the boundary of the metal and semiconductor material is insufficient to exceed the Schottky barrier in terms of converted energy. However, the hot hole or the hot electron may have a sufficient converted energy to exceed the Schottky barrier height during repetition of the reflection due to variance of the incident angle. If so, the hot hole or the hot electron can be injected into the semiconductor material. In this situation, each hot hole or each hot electron is injected into the semiconductor material and participates the electric currents. These phenomena are conducive to improvement in injection efficiency of hot hole (in the case of FIG. 2) or of hot electron (in the case of FIG. 3). In general, the thinner the metal is used, the higher in the injection efficiency the detector gets. However, if the metal is decreased in thickness over a certain value, the absorption of the infrared rays deteriorates, so that the metal should be optimized in view of quantum efficiency. For this reason, the Schottky-barrier infrared detector usually has a metal layer with the optimum value.
However, problems are encountered in the prior-art Schottky-barrier infrared detector as follows.
First, the metal film serving as the photoelectric converting region has a polycrystal structure, so that the metal film has a large number of scattering centers such as grain boundaries and lattice defects. If the semiconductor substrate is formed of single crystal silicon, an epitaxial silicide such as, for example, a platinous mono-silicide (PtSi) is usually used instead of the metal film because of its orientation. However, lattice mismatches tend to take place between the silicon substrate and the silicide film grown by an epitaxial technique, and, for this reason, grain boundaries and lattice defects are liable to be produced so as to reduce the mechanical stress due to the lattice mismatches. This means that there is no difference between the metal film and the silicide film. In other words, the hot holes or the hot electrons are liable to be scattered due to not only thermal vibrations of the crystal lattice but also the grain boundaries and the lattice defects, so that the hot holes or the hot electrons respectively lose parts of energies thereof during the traveling in the metal region or silicide film. Then, a problem is encountered in the prior-art Schottky-barrier infrared detector in short lifetime of the hot hole or the hot electron produced therein which in turn results in deterioration in injection efficiency of the hot hole or the hot electron.
Second, a Schottky contact is a kind of heterojunction, so that the boundary surface is inferior in comparison with a homojunction. This results in deterioration in injection efficiency of the hot hole from the metal film to the p-type semiconductor material or in injection efficiency of the hot electron from the metal film to the n-type semiconductor material.
Third, when the Schottky-barrier infrared detector is of the back illuminated type, each available ray should have a photon energy hv in a range given by the following inequality (1) EQU E.sub.SB &lt;hv&lt;Eg (Inequality 1)
where E.sub.SB is the Schottky-barrier height and Eg is energy bandgap of the semiconductor material. However, a metal has a large number of energy levels under the Fermi level occupied by electrons and, on the other hand, a large number of vacancy levels over the Fermi level, so that the photon energies are liable to be consumed with excitement of hot holes without sufficient energy levels to exceed the Schottky-barrier formed at the boundary surface between the metal and the p-type semiconductor material or with excitement of hot electrons without sufficient energy levels to exceed the Schottky-barrier formed at the boundary surface between the metal and the n-type semiconductor material. These undesirable phenomena take place upon absorption of the photon energy by an electron occupying an energy level Ee in the following range EQU Ef-E.sub.SB .ltoreq.Ee.ltoreq.Ef
where Ef is the Fermi level and E.sub.SB is the Schottky-barrier height between the metal and the p-type semiconductor material. If a Schottky barrier is formed between a metal and an n-type semiconductor material, the undesirable phenomena take place under an excitement of an electron occupying an energy level Ee' in the following range. EQU Ef-hv&lt;Ee'.ltoreq.Ef-hv+E.sub.SB
These undesirable phenomena are represented by the hot hole and the hot electron without arrows respectively extending toward the semiconductor materials in FIGS. 2 and 3.
It is sure that a few hot holes with insufficient energies or a few hot electrons with insufficient energies can be injected into the semiconductor material in virtue of the tunnel effect However, this injection efficiency is extremely small, so that a material amount the photon energies are consumed by the hot holes or the hot electrons with the energy levels in the above ranges without producing the electric currents representing the electric signal. This kind of photon energy loss becomes serious in a Schottky-barrier infrared detector of the back illuminated type having an optical cavity similar to that illustrated in FIG. 1.
Additionally, if a Schottky-barrier infrared detector has a semiconductor material with a wide energy bandgap, a wavelength of a detectable ray may occupy a position in the visible ray area as will be understood from the above mentioned inequality. Moreover, if a detector is of the front illuminated type or of the back illuminated type having an extremely narrow semiconductor layer allowing incident rays with energy levels larger than the energy bandgap of the semiconductor material to pass therethrough, the uppermost energy level of available rays may be larger in value than the energy bandgap Eg of the semiconductor material.
Fourth, it is difficult to form a Schottky-barrier with an arbitrary height because the Schottky-barrier height is almost determined by a combination of metal and semiconductor material. This means that a problem is encountered in determination of cutoff wavelength. It is well known in the art that a responsivity R of a Schottky-barrier infrared detector is given by the following equation ##EQU1## where C.sub.1 is a constant, 1 is the wavelength of a incident ray and c is the speed of light in vacuum. As will be clear from the above equation, the lower the Schottky-barrier it is formed, the longer cut-off wavelength it is selected. Then, the responsivity is increased in value. On the other hand, if a relatively low Schottky-barrier is formed, the hot holes or the hot electrons exceeding the Schottky-barrier are increased in number due to thermal excitement, thereby increasing the dark current as described hereinbefore. Then, it is necessary to use the Schottky-barrier infrared detector operative in a low temperature, and, for this reason, a problem is encountered in using a cooling unit. In other words, there is a trade-off between the cut-off wavelength and the operating temperature. This means that a cut-off wavelength should be selected in a reasonable low temperature for improvement, however a Schottky-barrier height strongly links with a combination of metal and semiconductor as described hereinbefore, so that adjustment of the Schottky-barrier height is impossible in so far as a suitable combination of metal and semiconductor material would be found by a designer.